1. Field of the Invention
This invention relates to a piezoelectric actuator, especially to a piezoelectric actuator for actuating a movable member such as valves in response to expansion and contraction of a piezoelectric element.
2. Prior Art
Recently, a piezoelectric element featuring high responsiveness in expanding action has been widely used as an actuator in various fields where quick responsiveness is required. For example, the piezoelectric element is used in a fuel injection valve of a fuel injection system so that fuel injection is performed with quick responsiveness to high revolution speed of an internal combustion engine. A fuel injection system utilizing a piezoelectric element has been proposed in Japan Published Unexamined patent application No. Sho 48-4823. A detailed explanation of this art is set forth with reference to FIG. 8.
In a fuel injection system A, a piezoelectric element A1 expands or contracts according to charging or discharging of electricity, thereby a piston A3 is moved up and down within a hydrualic cylinder A2. When the piston A3 is moved down, work oil A4 under the piston A3 is applied with pressure, thus, an upper plunger member A5 is lowered. Then, the upper plunger member A5 comes in contact with a lower plunger member A6 to push it down. As a result, a fuel injection nozzle A7 is opened to inject fuel. A belleville spring A8 is provided under the piston A3 in order to stabilize the piezoelectric element A1 by applying an initial load to the element A1.
The above-mentioned prior art includes some shortcomings set forth. The piezoelectric element A1 is covered with a casing A9. In this case, a certain space must be left between the piezoelectric element A1 and the casing A9 for maintaining sufficient insulation and cooling effects. Since the piezoelectric element A1 and the casing A9 are not in contact with each other, it is difficult to accurately position the element A1 to the casing A9. The element A1, therefore, is sometimes slantingly positioned within the casing A9. If the piezoelectric element A1 is actuated under the above-mentioned condition, a biased counter force is applied to the element A1 by the piston A3, which may result in cracking or sometimes severely damaging the element A1.
Moreover, since the piezoelectric element A1 consists of a number of laminated thin piezoelectric elements, the overall length of the piezoelectric element A1 sometimes varies with a minute error. The end surface of the piezoelectric element, therefore, is sometimes attached to the piston A3 out of perpendicular to the expanding/contracting direction of the piezoelectric element A1. As a result, when the piezoelectric element A1 is actuated, the element A1 is applied with a biased counter force which may lead to a breakdown of the element A1.
Furthermore, if the piezoelectric element A1 is slantingly attached or the element A1 itself includes a certain slant, the piston A3 is also applied with a biased force. Thus, the sliding portion of the piston A3 is apt to have a partial abrasion due to a defective sliding. In a worse case, the piston A3 may be disabled. There is still another problem. Since a space is formed between the piston A3 and the cylinder A2, the work oil A4 collected in a gap under the piston A3 is apt to leak into the piezoelectric element A1. In such a case, the pushing force of the piezoelectric element A1 is not sufficiently transmitted to the plunger A5.